Phenol and substituted phenols are important products in the chemical industry and are useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene relative to that of butenes is likely to increase, due to a developing shortage of propylene.
Thus, a process that uses butenes or higher alkenes instead of propylene as feed and that coproduces methyl ethyl ketone (MEK) or higher ketones, such as cyclohexanone, rather than acetone may be an attractive alternative route to the production of phenols. For example, there is a growing market for MEK, which is useful as a lacquer, a solvent and for dewaxing of lubricating oils. In addition, cyclohexanone is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam and nylon 6.
It is known that phenol and MEK can be produced from sec-butylbenzene, in a process where sec-butylbenzene is oxidized to obtain sec-butylbenzene hydroperoxide and the peroxide decomposed to the desired phenol and methyl ethyl ketone. An overview of such a process is described on pages 113-121 and 261-263 of Process Economics Report No. 22B entitled “Phenol”, published by the Stanford Research Institute in December 1977.
However, in comparison to cumene, oxidation of aromatic compounds substituted by branched alkyl groups having 4 or more carbon atoms, such as sec-butylbenzene, to the corresponding hydroperoxide requires higher temperatures and is very sensitive to the presence of impurities. For example, in the case of sec-butylbenzene containing 1% by weight of isobutylbenzene, the rate of formation of sec-butylbenzene hydroperoxide decreases to about 91% of that when the sec-butylbenzene is free of isobutylbenzene. Similarly, when the isobutylbenzene content is 1.65% by weight, the rate of oxidation decreases to about 86%; when the isobutylbenzene content is 2% by weight, the rate of oxidation decreases to about 84%; and when the isobutylbenzene content is 3.5% by weight, the rate of oxidation decreases to as low as about 82%.
Thus there is a need to find an oxidation process for producing for example C4+ alkyl aromatic hydroperoxides, and particularly sec-butylbenzene hydroperoxide, that is much less sensitive to the presence of impurities than the existing oxidation processes, and that allows efficient commercial scale production of phenol and MEK or higher ketones.
U.S. Pat. Nos. 6,852,893 (Creavis) and 6,720,462 (Creavis) describe methods for producing phenol by catalytic oxidation of alkylaromatic hydrocarbons to the corresponding hydroperoxide, and subsequent cleavage of the hydroperoxide to give phenol and a ketone. Catalytic oxidation takes place with oxygen, in the presence of a free radical initiator and a catalyst, typically an N-hydroxycarbodiimide catalyst, such as N-hydroxyphthalimide. Preferred substrates that may be oxidized by this process include cumene, cyclohexylbenzene, cyclododecylbenzene and sec-butylbenzene.
In addition, the article by Sheldon et al entitled “Organocatalytic Oxidations Mediated by Nitroxyl Radicals” in Adv. Synth. Catal., 2004, 346, pages 1051-1071 discloses that cyclohexylbenzene (CHB) can be oxidized to the 1-hydroperoxide with 97.6% selectivity at 32% CHB conversion at 100° C. in the presence of 0.5 mol % of a N-hydroxyphthalimide catalyst and 2 mol % of the product hydroperoxide as a free radical initiator.
However, while N-hydroxycarbodiimides have shown activity and selectivity for the oxidation of alkylaromatic compounds to their corresponding hydroperoxides, they suffer from the problems inherent in any homogeneous catalyst in that they tend to be removed from the reaction zone with the product effluent and so must be separated from the product effluent. There is therefore significant interest in developing a heterogeneous oxidation catalyst for producing alkylaromatic hydroperoxides provided adequate activity and selectivity can still be maintained.
In our International Patent Publication No. WO 06/15826 we have described a process for producing phenol and methyl ethyl ketone, in which a feed comprising benzene and a C4 alkylating agent is contacted under alkylation conditions with catalyst comprising zeolite beta or a molecular sieve having an X-ray diffraction pattern including d-spacing maxima at 12.4±0.25, 6.9±0.15, 3.57±0.07 and 3.42±0.07 Ångstrom to produce an alkylation effluent comprising sec-butylbenzene. The sec-butylbenzene is then oxidized to produce a hydroperoxide and the hydroperoxide is cleaved to produce the desired phenol and methyl ethyl ketone. Although homogeneous catalysts, such as N-hydroxy substituted cyclic imides, are disclosed as suitable for the oxidation reaction, preferred catalysts are said to be heterogeneous catalysts, such as the oxo (hydroxo) bridged tetranuclear manganese complexes described in U.S. Pat. No. 5,183,945 and U.S. Pat. No. 5,922,920.
Another class of compounds that have been widely described as catalysts, including oxidation catalysts, are polyoxometalates (“POM's”), which are described in Pope et al., Heteropoly and Isopoly Oxometalates: Inorganic Chemistry Concepts, Springer-Verlag, New York (1983), incorporated herein by reference. Pope et al. and others have described numerous uses of POM's in catalysis such as oxidation of propylene and isobutylene to acrylic and methacrylic acids, oxidation of aromatic hydrocarbons; olefin polymerization; ammoxidation; oxidation of crotonaldehyde or butadiene to furan; dehydration of alcohols; oxidative coupling of alkyl benzenes or heterocycles; epoxidation; and hydrodesulfurization.
According to the invention, it has now been found that certain tungsten-containing polyoxometalates show activity and selectivity as catalysts in the oxidation of secondary alkyl substituted benzenes, including sec-butylbenzene and cyclohexylbenzene, to the corresponding hydroperoxides.